The class creates and directly depends on the MyDependency instance. Code dependencies (such as the previous example) are problematic and should be avoided for the following reasons:

To replace MyDependency with a different implementation, the class must be modified.

If MyDependency has dependencies, they must be configured by the class. In a large project with multiple classes depending on MyDependency, the configuration code becomes scattered across the app.

This implementation is difficult to unit test. The app should use a mock or stub MyDependency class, which isn't possible with this approach.

Dependency injection addresses these problems through:

The use of an interface to abstract the dependency implementation.

Registration of the dependency in a service container. ASP.NET Core provides a built-in service container, IServiceProvider. Services are registered in the app's Startup.ConfigureServices method.

Injection of the service into the constructor of the class where it's used. The framework takes on the responsibility of creating an instance of the dependency and disposing of it when it's no longer needed.

In the sample app, the IMyDependency interface defines a method that the service provides to the app:

MyDependency requests an ILogger<TCategoryName> in its constructor. It's not unusual to use dependency injection in a chained fashion. Each requested dependency in turn requests its own dependencies. The container resolves the dependencies in the graph and returns the fully resolved service. The collective set of dependencies that must be resolved is typically referred to as a dependency tree, dependency graph, or object graph.

IMyDependency and ILogger<TCategoryName> must be registered in the service container. IMyDependency is registered in Startup.ConfigureServices. ILogger<TCategoryName> is registered by the logging abstractions infrastructure, so it's a framework-provided service registered by default by the framework.

In the sample app, the IMyDependency service is registered with the concrete type MyDependency. The registration scopes the service lifetime to the lifetime of a single request. Service lifetimes are described later in this topic.

Each services.Add{SERVICE_NAME} extension method adds (and potentially configures) services. For example, services.AddMvc() adds the services Razor Pages and MVC require. We recommended that apps follow this convention. Place extension methods in the Microsoft.Extensions.DependencyInjection namespace to encapsulate groups of service registrations.

If the service's constructor requires a primitive, such as a string, the primitive can be injected by using configuration or the options pattern:

An instance of the service is requested via the constructor of a class where the service is used and assigned to a private field. The field is used to access the service as necessary throughout the class.

In the sample app, the IMyDependency instance is requested and used to call the service's WriteMessage method:

Framework-provided services

The Startup.ConfigureServices method is responsible for defining the services the app uses, including platform features, such as Entity Framework Core and ASP.NET Core MVC. Initially, the IServiceCollection provided to ConfigureServices has the following services defined (depending on how the host was configured):

When a service collection extension method is available to register a service (and its dependent services, if required), the convention is to use a single Add{SERVICE_NAME} extension method to register all of the services required by that service. The following code is an example of how to add additional services to the container using the extension methods AddDbContext, AddIdentity, and AddMvc:

Service lifetimes

Choose an appropriate lifetime for each registered service. ASP.NET Core services can be configured with the following lifetimes:

Transient

Transient lifetime services are created each time they're requested. This lifetime works best for lightweight, stateless services.

Scoped

Scoped lifetime services are created once per request.

Warning

When using a scoped service in a middleware, inject the service into the Invoke or InvokeAsync method. Don't inject via constructor injection because it forces the service to behave like a singleton. For more information, see ASP.NET Core Middleware.

Singleton

Singleton lifetime services are created the first time they're requested (or when ConfigureServices is run and an instance is specified with the service registration). Every subsequent request uses the same instance. If the app requires singleton behavior, allowing the service container to manage the service's lifetime is recommended. Don't implement the singleton design pattern and provide user code to manage the object's lifetime in the class.

Warning

It's dangerous to resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests.

Constructor injection behavior

Services can be resolved by two mechanisms:

IServiceProvider

ActivatorUtilities – Permits object creation without service registration in the dependency injection container. ActivatorUtilities is used with user-facing abstractions, such as Tag Helpers, MVC controllers, and model binders.

Constructors can accept arguments that aren't provided by dependency injection, but the arguments must assign default values.

When services are resolved by IServiceProvider or ActivatorUtilities, constructor injection requires a public constructor.

When services are resolved by ActivatorUtilities, constructor injection requires that only one applicable constructor exists. Constructor overloads are supported, but only one overload can exist whose arguments can all be fulfilled by dependency injection.

Entity Framework contexts

Entity Framework contexts should be added to the service container using the scoped lifetime. This is handled automatically with a call to the AddDbContext method when registering the database context. Services that use the database context should also use the scoped lifetime.

Lifetime and registration options

To demonstrate the difference between the lifetime and registration options, consider the following interfaces that represent tasks as an operation with a unique identifier, OperationId. Depending on how the lifetime of an operations service is configured for the following interfaces, the container provides either the same or a different instance of the service when requested by a class:

An OperationService is registered that depends on each of the other Operation types. When OperationService is requested via dependency injection, it receives either a new instance of each service or an existing instance based on the lifetime of the dependent service.

If transient services are created when requested, the OperationId of the IOperationTransient service is different than the OperationId of the OperationService. OperationService receives a new instance of the IOperationTransient class. The new instance yields a different OperationId.

If scoped services are created per request, the OperationId of the IOperationScoped service is the same as that of OperationService within a request. Across requests, both services share a different OperationId value.

If singleton and singleton-instance services are created once and used across all requests and all services, the OperationId is constant across all service requests.

The IOperationSingletonInstance service is using a specific instance with a known ID of Guid.Empty. It's clear when this type is in use (its GUID is all zeroes).

The sample app demonstrates object lifetimes within and between individual requests. The sample app's IndexModel requests each kind of IOperation type and the OperationService. The page then displays all of the page model class's and service's OperationId values through property assignments:

The sample app demonstrates object lifetimes within and between individual requests. The sample app includes an OperationsController that requests each kind of IOperation type and the OperationService. The Index action sets the services into the ViewBag for display of the service's OperationId values:

Observe which of the OperationId values vary within a request and between requests:

Transient objects are always different. Note that the transient OperationId value for both the first and second requests are different for both OperationService operations and across requests. A new instance is provided to each service and request.

Scoped objects are the same within a request but different across requests.

Singleton objects are the same for every object and every request regardless of whether an Operation instance is provided in ConfigureServices.

Call services from main

Create an IServiceScope with IServiceScopeFactory.CreateScope to resolve a scoped service within the app's scope. This approach is useful to access a scoped service at startup to run initialization tasks. The following example shows how to obtain a context for the MyScopedService in Program.Main:

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when BuildServiceProvider is called.

Request Services

Request Services represent the services configured and requested as part of the app. When the objects specify dependencies, these are satisfied by the types found in RequestServices, not ApplicationServices.

Generally, the app shouldn't use these properties directly. Instead, request the types that classes require via class constructors and allow the framework inject the dependencies. This yields classes that are easier to test.

If a class seems to have too many injected dependencies, this is generally a sign that the class has too many responsibilities and is violating the Single Responsibility Principle (SRP). Attempt to refactor the class by moving some of its responsibilities into a new class. Keep in mind that Razor Pages page model classes and MVC controller classes should focus on UI concerns. Business rules and data access implementation details should be kept in classes appropriate to these separate concerns.

Disposal of services

The container calls Dispose for the IDisposable types it creates. If an instance is added to the container by user code, it isn't disposed automatically.

In ASP.NET Core 1.0, the container calls dispose on allIDisposable objects, including those it didn't create.

Default service container replacement

The built-in service container is meant to serve the needs of the framework and most consumer apps. We recommend using the built-in container unless you need a specific feature that it doesn't support. Some of the features supported in 3rd party containers not found in the built-in container:

At runtime, Autofac is used to resolve types and inject dependencies. To learn more about using Autofac with ASP.NET Core, see the Autofac documentation.

Thread safety

Singleton services need to be thread safe. If a singleton service has a dependency on a transient service, the transient service may also need to be thread safe depending how it's used by the singleton.

Recommendations

async/await and Task based service resolution is not supported. C# does not support asynchronous constructors, therefore the recommended pattern is to use asynchronous methods after synchronously resolving the service.

Avoid storing data and configuration directly in the service container. For example, a user's shopping cart shouldn't typically be added to the service container. Configuration should use the options pattern. Similarly, avoid "data holder" objects that only exist to allow access to some other object. It's better to request the actual item via DI.

Avoid using the service locator pattern. For example, don't invoke GetService to obtain a service instance when you can use DI instead. Another service locator variation to avoid is injecting a factory that resolves dependencies at runtime. Both of these practices mix Inversion of Control strategies.